A manual transmission (MT) carrying out a gear shift operation manually has an input shaft to which a plurality of drive gears are provided, and an output shaft to which a plurality of driven gears forming transmission gear trains together with the drive gears are provided. An engine power is input to the input shaft via an input clutch, and the output shaft is connected to a drive wheel. In this MT, a gear shift operation, that is, a shift change is carried out by disconnecting the input clutch for changing gears, thereafter switching by an engagement clutch such as a synchro-mesh mechanism or the like into one of the gear trains to carry out the power transmission by selecting a gear train from a plurality of transmission gear trains, and connecting the input clutch thereto.
In the case where the shift change and the clutch operation are driven by a hydraulic actuator and where the hydraulic actuator is electronically controlled, an automatic transmission having a structure on the basis of the manual transmission system can be obtained. This type of automated manual transmission (hereinafter, refer briefly to as an AMT) having a plurality of the transmission gear trains has advantages that a weight saving can be easily attained by reducing a number of parts in comparison with an ordinary torque converter type automatic transmission (AT) having planetary gears in an automatic transmission mechanism, and that a power transmitting efficiency becomes high.
In this AMT, when carrying out the gear shift by an electronic control, the gear shift operation is carried out by disconnecting the input clutch provided between a crankshaft of the engine and the input shaft of the transmission so as to temporarily disconnect the power, in accordance with an automatic gear shift schedule (or pattern) which is previously determined on the basis of a vehicle speed, a throttle opening degree of the engine and the like, so that no torque transmitting condition is instantly generated when shifting the gear. Accordingly, for example, in the case where the transmission of the engine power to the transmission system is disconnected for a moment when shifting up to a second speed from a first speed in which there is a large difference in a gear ratio, the drive force change is large even if structured for the drive force to be smoothly changed between before and after the gear shifts. Thus, the driver feels the driving speed reduced as well as torque loss during the gear shift, so that the driver may feel uncomfortable.
For the purpose of avoiding such torque disconnection while the gear shift operation, the automatic transmission provided with a hydraulic multiple disc type bypass clutch is disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-65199.
This publication discloses an automatic transmission in which first to fifth speed gear trains are provided in input and output shafts; switching between the first and second speed gear trains and switching between the third and fourth speed gear trains are respectively carried out by using the same synchronizer mechanism as that of the conventional manual transmission; and transmitting the power and disconnecting the torque transmission on the basis of the fifth speed gear train are carried out by a bypass clutch. In this automatic transmission, a torque reduction from the input shaft to the output shaft is prevented by transmitting the power from the fifth speed gear train by connecting the bypass clutch at a time of the gear shift. However, since the bypass clutch is provided in the fifth speed gear train, a torque transmitting capacity to the output shaft from the input shaft via the bypass clutch corresponds to a gear ratio of the fifth speed.
Accordingly, at a time of shifting gears from the first speed to the second speed, the torque capacity which can be transmitted to the output shaft from the input shaft via the bypass clutch for the purpose of preventing the torque disconnection becomes a drive force applied by the gear shift ratio of the fifth speed. And the gear ratio difference is large between this speed and the second speed even by adding the torque increase generated by an inertia moment in the input side of the bypass clutch, so that it is impossible to sufficiently compensate the difference in the driving force change between the both. If the difference is large, not only the gear shift shock is generated so as to cause a reduction in a gear shift quality of the automatic transmission, but also the torque is reduced during the gear shift even when an acceleration is required, so that the driver feels uncomfortable. In the same manner, when shifting the gears from the second speed to the third speed, it is also impossible to sufficiently compensate the difference between the transmission torque transmitted via the bypass clutch and the drive force change caused by the gear shift ratio difference from the second speed to the third speed, even by adding the torque increase generated by the inertia moment mentioned above, so that a gear shift shock is generated and a gear shift quality reduction is caused.
In order to improve a fuel consumption and a quietness during a high speed cruising and applying a large start drive force and a large traction force to a four-wheel drive vehicle, there is a case of mounting on a vehicle an automatic transmission provided with a gear ratio in which a number of shift ranges is set equal to or more than five ranges, for example, six ranges, or further one range of extra low range gear ratio is added to the transmission gear ranges, thereby improving a quality of the vehicle. When increasing the number of the transmission gear ranges as mentioned above, a difference between the lowest range (i.e. first speed) and the highest range (e.g. sixth speed) becomes large, so that in the vehicle, the gear shift quality mentioned above is further lowered. In particular, taking into consideration of the automatic transmission applied to a utility vehicle and a heavy vehicle, this tendency becomes further significant in accordance that the difference between the gear shift rates becomes large.
On the other hand, an automatic transmission having such a structure that two shift clutches are provided between the input shaft and the output shaft is proposed and disclosed in Japanese Patent Application Laid-Open No. 4-262166. In this automatic transmission, one shift clutch transmits the power to transmission gear trains of second and fourth speed of even (numbered) transmission gear ranges, and another shift clutch transmits the power to the transmission gear trains of first, third and fifth speed of odd (numbered) transmission gear ranges, whereby the gear shift is carried out by changing between two gear shift clutches, that is, switching the gear shift. For example, at a time of shifting up to the second speed from the first speed, since the gear shift clutch in a side of the even range is in a disconnected state. Thus the synchronizer for the second speed of the even range previously operates a selectively operable synchro-sleeve in a shifting manner so as to engage the second speed synchronizer with the output shaft. And then the synchronizer changes the gear shift clutch in the side of the odd range constituting the transmission gear train of the first speed and the gear shift clutch in the side of the even range, thereby achieving the gear shift operation.
Further, in this transmission, two gear shift clutches are provided between the engine and the input shaft, and the transmission gear trains are roughly classified into transmission gear trains of odd or even gear shift ranges connected so as to be capable of transmitting the power from one clutch to the first input shaft, and transmission gear trains of odd or even gear shift ranges connected so as to be capable of transmitting the power from another clutch to the second input shaft. Further, the synchronizer is provided in each of the transmission gear trains.
However, since this automatic transmission shift gears by alternately switching the shift clutch in the odd range side and the shift clutch in the even range side, it is necessary to shift the gears via the transmission gear range on the way in the case of shifting up from the first speed to the fifth speed. Accordingly, it is hard to shift the gears in a jumped (or skipped) manner such as the gear shift from the odd range to the odd range and the gear shift from the even range to the even range, for example, the gear shift from the first speed to the third speed and the gear shift from the second speed to the fourth speed. In this case, since the speed increase and the speed reduction are carried out while passing via the transmission gear range on the way, there are generated problems such that a time for shifting gears becomes generally long, the driver feels a stress in the gear shift during an acceleration for passing over, and the like. In particular, a difference between an intention of the driver and a motion of the vehicle is frequently generated at a time ofjumped kick-down gear shift from the maximum transmission gear range. This phenomenon appears as a significant phenomenon in a travel on a mountain road having a lot of curves and ups and downs or for a vehicle having a comparatively small displacement and less power.
On the contrary, since the structure of the transmission gear train is the structure which is clearly classified into the odd range train group and the even range train group, there is generated a problem that a design freedom is not obtained in the arrangement of the gears in the transmission main body and the layout of the operation system. In particular, if it is desired to wholly and appropriately use the basic structure of the manual transmission, there is generated the problem that the same mechanism can be used only by arranging the manual transmission in such a manner that the transmission gear trains are separated into the even transmission gear trains and the odd transmission gear trains.